diff options
Diffstat (limited to 'Documentation/technical')
| -rw-r--r-- | Documentation/technical/api-argv-array.txt | 2 | ||||
| -rw-r--r-- | Documentation/technical/api-builtin.txt | 73 | ||||
| -rw-r--r-- | Documentation/technical/api-config.txt | 2 | ||||
| -rw-r--r-- | Documentation/technical/api-directory-listing.txt | 33 | ||||
| -rw-r--r-- | Documentation/technical/api-gitattributes.txt | 86 | ||||
| -rw-r--r-- | Documentation/technical/api-hashmap.txt | 287 | ||||
| -rw-r--r-- | Documentation/technical/api-oid-array.txt (renamed from Documentation/technical/api-sha1-array.txt) | 44 | ||||
| -rw-r--r-- | Documentation/technical/api-parse-options.txt | 13 | ||||
| -rw-r--r-- | Documentation/technical/api-ref-iteration.txt | 7 | ||||
| -rw-r--r-- | Documentation/technical/api-string-list.txt | 209 | ||||
| -rw-r--r-- | Documentation/technical/api-tree-walking.txt | 6 | ||||
| -rw-r--r-- | Documentation/technical/hash-function-transition.txt | 797 | ||||
| -rw-r--r-- | Documentation/technical/index-format.txt | 19 | ||||
| -rw-r--r-- | Documentation/technical/pack-protocol.txt | 41 | ||||
| -rw-r--r-- | Documentation/technical/trivial-merge.txt | 4 |
15 files changed, 974 insertions, 649 deletions
diff --git a/Documentation/technical/api-argv-array.txt b/Documentation/technical/api-argv-array.txt index cfc063018c..870c8edbfb 100644 --- a/Documentation/technical/api-argv-array.txt +++ b/Documentation/technical/api-argv-array.txt @@ -8,7 +8,7 @@ always NULL-terminated at the element pointed to by `argv[argc]`. This makes the result suitable for passing to functions expecting to receive argv from main(), or the link:api-run-command.html[run-command API]. -The link:api-string-list.html[string-list API] is similar, but cannot be +The string-list API (documented in string-list.h) is similar, but cannot be used for these purposes; instead of storing a straight string pointer, it contains an item structure with a `util` field that is not compatible with the traditional argv interface. diff --git a/Documentation/technical/api-builtin.txt b/Documentation/technical/api-builtin.txt deleted file mode 100644 index 22a39b9299..0000000000 --- a/Documentation/technical/api-builtin.txt +++ /dev/null @@ -1,73 +0,0 @@ -builtin API -=========== - -Adding a new built-in ---------------------- - -There are 4 things to do to add a built-in command implementation to -Git: - -. Define the implementation of the built-in command `foo` with - signature: - - int cmd_foo(int argc, const char **argv, const char *prefix); - -. Add the external declaration for the function to `builtin.h`. - -. Add the command to the `commands[]` table defined in `git.c`. - The entry should look like: - - { "foo", cmd_foo, <options> }, -+ -where options is the bitwise-or of: - -`RUN_SETUP`:: - If there is not a Git directory to work on, abort. If there - is a work tree, chdir to the top of it if the command was - invoked in a subdirectory. If there is no work tree, no - chdir() is done. - -`RUN_SETUP_GENTLY`:: - If there is a Git directory, chdir as per RUN_SETUP, otherwise, - don't chdir anywhere. - -`USE_PAGER`:: - - If the standard output is connected to a tty, spawn a pager and - feed our output to it. - -`NEED_WORK_TREE`:: - - Make sure there is a work tree, i.e. the command cannot act - on bare repositories. - This only makes sense when `RUN_SETUP` is also set. - -. Add `builtin/foo.o` to `BUILTIN_OBJS` in `Makefile`. - -Additionally, if `foo` is a new command, there are 3 more things to do: - -. Add tests to `t/` directory. - -. Write documentation in `Documentation/git-foo.txt`. - -. Add an entry for `git-foo` to `command-list.txt`. - -. Add an entry for `/git-foo` to `.gitignore`. - - -How a built-in is called ------------------------- - -The implementation `cmd_foo()` takes three parameters, `argc`, `argv, -and `prefix`. The first two are similar to what `main()` of a -standalone command would be called with. - -When `RUN_SETUP` is specified in the `commands[]` table, and when you -were started from a subdirectory of the work tree, `cmd_foo()` is called -after chdir(2) to the top of the work tree, and `prefix` gets the path -to the subdirectory the command started from. This allows you to -convert a user-supplied pathname (typically relative to that directory) -to a pathname relative to the top of the work tree. - -The return value from `cmd_foo()` becomes the exit status of the -command. diff --git a/Documentation/technical/api-config.txt b/Documentation/technical/api-config.txt index 20741f345e..9a778b0cad 100644 --- a/Documentation/technical/api-config.txt +++ b/Documentation/technical/api-config.txt @@ -186,7 +186,7 @@ parsing is successful, the return value is the result. Same as `git_config_bool`, except that integers are returned as-is, and an `is_bool` flag is unset. -`git_config_maybe_bool`:: +`git_parse_maybe_bool`:: Same as `git_config_bool`, except that it returns -1 on error rather than dying. diff --git a/Documentation/technical/api-directory-listing.txt b/Documentation/technical/api-directory-listing.txt index 7f8e78d916..7fae00f44f 100644 --- a/Documentation/technical/api-directory-listing.txt +++ b/Documentation/technical/api-directory-listing.txt @@ -22,16 +22,41 @@ The notable options are: `flags`:: - A bit-field of options (the `*IGNORED*` flags are mutually exclusive): + A bit-field of options: `DIR_SHOW_IGNORED`::: - Return just ignored files in `entries[]`, not untracked files. + Return just ignored files in `entries[]`, not untracked + files. This flag is mutually exclusive with + `DIR_SHOW_IGNORED_TOO`. `DIR_SHOW_IGNORED_TOO`::: - Similar to `DIR_SHOW_IGNORED`, but return ignored files in `ignored[]` - in addition to untracked files in `entries[]`. + Similar to `DIR_SHOW_IGNORED`, but return ignored files in + `ignored[]` in addition to untracked files in + `entries[]`. This flag is mutually exclusive with + `DIR_SHOW_IGNORED`. + +`DIR_KEEP_UNTRACKED_CONTENTS`::: + + Only has meaning if `DIR_SHOW_IGNORED_TOO` is also set; if this is set, the + untracked contents of untracked directories are also returned in + `entries[]`. + +`DIR_SHOW_IGNORED_TOO_MODE_MATCHING`::: + + Only has meaning if `DIR_SHOW_IGNORED_TOO` is also set; if + this is set, returns ignored files and directories that match + an exclude pattern. If a directory matches an exclude pattern, + then the directory is returned and the contained paths are + not. A directory that does not match an exclude pattern will + not be returned even if all of its contents are ignored. In + this case, the contents are returned as individual entries. ++ +If this is set, files and directories that explicity match an ignore +pattern are reported. Implicity ignored directories (directories that +do not match an ignore pattern, but whose contents are all ignored) +are not reported, instead all of the contents are reported. `DIR_COLLECT_IGNORED`::: diff --git a/Documentation/technical/api-gitattributes.txt b/Documentation/technical/api-gitattributes.txt index 2602668677..e7cbb7c13a 100644 --- a/Documentation/technical/api-gitattributes.txt +++ b/Documentation/technical/api-gitattributes.txt @@ -16,10 +16,15 @@ Data Structure of no interest to the calling programs. The name of the attribute can be retrieved by calling `git_attr_name()`. -`struct git_attr_check`:: +`struct attr_check_item`:: - This structure represents a set of attributes to check in a call - to `git_check_attr()` function, and receives the results. + This structure represents one attribute and its value. + +`struct attr_check`:: + + This structure represents a collection of `attr_check_item`. + It is passed to `git_check_attr()` function, specifying the + attributes to check, and receives their values. Attribute Values @@ -27,7 +32,7 @@ Attribute Values An attribute for a path can be in one of four states: Set, Unset, Unspecified or set to a string, and `.value` member of `struct -git_attr_check` records it. There are three macros to check these: +attr_check_item` records it. There are three macros to check these: `ATTR_TRUE()`:: @@ -48,49 +53,51 @@ value of the attribute for the path. Querying Specific Attributes ---------------------------- -* Prepare an array of `struct git_attr_check` to define the list of - attributes you would want to check. To populate this array, you would - need to define necessary attributes by calling `git_attr()` function. +* Prepare `struct attr_check` using attr_check_initl() + function, enumerating the names of attributes whose values you are + interested in, terminated with a NULL pointer. Alternatively, an + empty `struct attr_check` can be prepared by calling + `attr_check_alloc()` function and then attributes you want to + ask about can be added to it with `attr_check_append()` + function. * Call `git_check_attr()` to check the attributes for the path. -* Inspect `git_attr_check` structure to see how each of the attribute in - the array is defined for the path. +* Inspect `attr_check` structure to see how each of the + attribute in the array is defined for the path. Example ------- -To see how attributes "crlf" and "indent" are set for different paths. +To see how attributes "crlf" and "ident" are set for different paths. -. Prepare an array of `struct git_attr_check` with two elements (because - we are checking two attributes). Initialize their `attr` member with - pointers to `struct git_attr` obtained by calling `git_attr()`: +. Prepare a `struct attr_check` with two elements (because + we are checking two attributes): ------------ -static struct git_attr_check check[2]; +static struct attr_check *check; static void setup_check(void) { - if (check[0].attr) + if (check) return; /* already done */ - check[0].attr = git_attr("crlf"); - check[1].attr = git_attr("ident"); + check = attr_check_initl("crlf", "ident", NULL); } ------------ -. Call `git_check_attr()` with the prepared array of `struct git_attr_check`: +. Call `git_check_attr()` with the prepared `struct attr_check`: ------------ const char *path; setup_check(); - git_check_attr(path, ARRAY_SIZE(check), check); + git_check_attr(path, check); ------------ -. Act on `.value` member of the result, left in `check[]`: +. Act on `.value` member of the result, left in `check->items[]`: ------------ - const char *value = check[0].value; + const char *value = check->items[0].value; if (ATTR_TRUE(value)) { The attribute is Set, by listing only the name of the @@ -109,20 +116,39 @@ static void setup_check(void) } ------------ +To see how attributes in argv[] are set for different paths, only +the first step in the above would be different. + +------------ +static struct attr_check *check; +static void setup_check(const char **argv) +{ + check = attr_check_alloc(); + while (*argv) { + struct git_attr *attr = git_attr(*argv); + attr_check_append(check, attr); + argv++; + } +} +------------ + Querying All Attributes ----------------------- To get the values of all attributes associated with a file: -* Call `git_all_attrs()`, which returns an array of `git_attr_check` - structures. +* Prepare an empty `attr_check` structure by calling + `attr_check_alloc()`. + +* Call `git_all_attrs()`, which populates the `attr_check` + with the attributes attached to the path. -* Iterate over the `git_attr_check` array to examine the attribute - names and values. The name of the attribute described by a - `git_attr_check` object can be retrieved via - `git_attr_name(check[i].attr)`. (Please note that no items will be - returned for unset attributes, so `ATTR_UNSET()` will return false - for all returned `git_array_check` objects.) +* Iterate over the `attr_check.items[]` array to examine + the attribute names and values. The name of the attribute + described by a `attr_check.items[]` object can be retrieved via + `git_attr_name(check->items[i].attr)`. (Please note that no items + will be returned for unset attributes, so `ATTR_UNSET()` will return + false for all returned `attr_check.items[]` objects.) -* Free the `git_array_check` array. +* Free the `attr_check` struct by calling `attr_check_free()`. diff --git a/Documentation/technical/api-hashmap.txt b/Documentation/technical/api-hashmap.txt deleted file mode 100644 index a3f020cd9e..0000000000 --- a/Documentation/technical/api-hashmap.txt +++ /dev/null @@ -1,287 +0,0 @@ -hashmap API -=========== - -The hashmap API is a generic implementation of hash-based key-value mappings. - -Data Structures ---------------- - -`struct hashmap`:: - - The hash table structure. Members can be used as follows, but should - not be modified directly: -+ -The `size` member keeps track of the total number of entries (0 means the -hashmap is empty). -+ -`tablesize` is the allocated size of the hash table. A non-0 value indicates -that the hashmap is initialized. It may also be useful for statistical purposes -(i.e. `size / tablesize` is the current load factor). -+ -`cmpfn` stores the comparison function specified in `hashmap_init()`. In -advanced scenarios, it may be useful to change this, e.g. to switch between -case-sensitive and case-insensitive lookup. - -`struct hashmap_entry`:: - - An opaque structure representing an entry in the hash table, which must - be used as first member of user data structures. Ideally it should be - followed by an int-sized member to prevent unused memory on 64-bit - systems due to alignment. -+ -The `hash` member is the entry's hash code and the `next` member points to the -next entry in case of collisions (i.e. if multiple entries map to the same -bucket). - -`struct hashmap_iter`:: - - An iterator structure, to be used with hashmap_iter_* functions. - -Types ------ - -`int (*hashmap_cmp_fn)(const void *entry, const void *entry_or_key, const void *keydata)`:: - - User-supplied function to test two hashmap entries for equality. Shall - return 0 if the entries are equal. -+ -This function is always called with non-NULL `entry` / `entry_or_key` -parameters that have the same hash code. When looking up an entry, the `key` -and `keydata` parameters to hashmap_get and hashmap_remove are always passed -as second and third argument, respectively. Otherwise, `keydata` is NULL. - -Functions ---------- - -`unsigned int strhash(const char *buf)`:: -`unsigned int strihash(const char *buf)`:: -`unsigned int memhash(const void *buf, size_t len)`:: -`unsigned int memihash(const void *buf, size_t len)`:: - - Ready-to-use hash functions for strings, using the FNV-1 algorithm (see - http://www.isthe.com/chongo/tech/comp/fnv). -+ -`strhash` and `strihash` take 0-terminated strings, while `memhash` and -`memihash` operate on arbitrary-length memory. -+ -`strihash` and `memihash` are case insensitive versions. - -`unsigned int sha1hash(const unsigned char *sha1)`:: - - Converts a cryptographic hash (e.g. SHA-1) into an int-sized hash code - for use in hash tables. Cryptographic hashes are supposed to have - uniform distribution, so in contrast to `memhash()`, this just copies - the first `sizeof(int)` bytes without shuffling any bits. Note that - the results will be different on big-endian and little-endian - platforms, so they should not be stored or transferred over the net. - -`void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, size_t initial_size)`:: - - Initializes a hashmap structure. -+ -`map` is the hashmap to initialize. -+ -The `equals_function` can be specified to compare two entries for equality. -If NULL, entries are considered equal if their hash codes are equal. -+ -If the total number of entries is known in advance, the `initial_size` -parameter may be used to preallocate a sufficiently large table and thus -prevent expensive resizing. If 0, the table is dynamically resized. - -`void hashmap_free(struct hashmap *map, int free_entries)`:: - - Frees a hashmap structure and allocated memory. -+ -`map` is the hashmap to free. -+ -If `free_entries` is true, each hashmap_entry in the map is freed as well -(using stdlib's free()). - -`void hashmap_entry_init(void *entry, unsigned int hash)`:: - - Initializes a hashmap_entry structure. -+ -`entry` points to the entry to initialize. -+ -`hash` is the hash code of the entry. -+ -The hashmap_entry structure does not hold references to external resources, -and it is safe to just discard it once you are done with it (i.e. if -your structure was allocated with xmalloc(), you can just free(3) it, -and if it is on stack, you can just let it go out of scope). - -`void *hashmap_get(const struct hashmap *map, const void *key, const void *keydata)`:: - - Returns the hashmap entry for the specified key, or NULL if not found. -+ -`map` is the hashmap structure. -+ -`key` is a hashmap_entry structure (or user data structure that starts with -hashmap_entry) that has at least been initialized with the proper hash code -(via `hashmap_entry_init`). -+ -If an entry with matching hash code is found, `key` and `keydata` are passed -to `hashmap_cmp_fn` to decide whether the entry matches the key. - -`void *hashmap_get_from_hash(const struct hashmap *map, unsigned int hash, const void *keydata)`:: - - Returns the hashmap entry for the specified hash code and key data, - or NULL if not found. -+ -`map` is the hashmap structure. -+ -`hash` is the hash code of the entry to look up. -+ -If an entry with matching hash code is found, `keydata` is passed to -`hashmap_cmp_fn` to decide whether the entry matches the key. The -`entry_or_key` parameter points to a bogus hashmap_entry structure that -should not be used in the comparison. - -`void *hashmap_get_next(const struct hashmap *map, const void *entry)`:: - - Returns the next equal hashmap entry, or NULL if not found. This can be - used to iterate over duplicate entries (see `hashmap_add`). -+ -`map` is the hashmap structure. -+ -`entry` is the hashmap_entry to start the search from, obtained via a previous -call to `hashmap_get` or `hashmap_get_next`. - -`void hashmap_add(struct hashmap *map, void *entry)`:: - - Adds a hashmap entry. This allows to add duplicate entries (i.e. - separate values with the same key according to hashmap_cmp_fn). -+ -`map` is the hashmap structure. -+ -`entry` is the entry to add. - -`void *hashmap_put(struct hashmap *map, void *entry)`:: - - Adds or replaces a hashmap entry. If the hashmap contains duplicate - entries equal to the specified entry, only one of them will be replaced. -+ -`map` is the hashmap structure. -+ -`entry` is the entry to add or replace. -+ -Returns the replaced entry, or NULL if not found (i.e. the entry was added). - -`void *hashmap_remove(struct hashmap *map, const void *key, const void *keydata)`:: - - Removes a hashmap entry matching the specified key. If the hashmap - contains duplicate entries equal to the specified key, only one of - them will be removed. -+ -`map` is the hashmap structure. -+ -`key` is a hashmap_entry structure (or user data structure that starts with -hashmap_entry) that has at least been initialized with the proper hash code -(via `hashmap_entry_init`). -+ -If an entry with matching hash code is found, `key` and `keydata` are -passed to `hashmap_cmp_fn` to decide whether the entry matches the key. -+ -Returns the removed entry, or NULL if not found. - -`void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter)`:: -`void *hashmap_iter_next(struct hashmap_iter *iter)`:: -`void *hashmap_iter_first(struct hashmap *map, struct hashmap_iter *iter)`:: - - Used to iterate over all entries of a hashmap. Note that it is - not safe to add or remove entries to the hashmap while - iterating. -+ -`hashmap_iter_init` initializes a `hashmap_iter` structure. -+ -`hashmap_iter_next` returns the next hashmap_entry, or NULL if there are no -more entries. -+ -`hashmap_iter_first` is a combination of both (i.e. initializes the iterator -and returns the first entry, if any). - -`const char *strintern(const char *string)`:: -`const void *memintern(const void *data, size_t len)`:: - - Returns the unique, interned version of the specified string or data, - similar to the `String.intern` API in Java and .NET, respectively. - Interned strings remain valid for the entire lifetime of the process. -+ -Can be used as `[x]strdup()` or `xmemdupz` replacement, except that interned -strings / data must not be modified or freed. -+ -Interned strings are best used for short strings with high probability of -duplicates. -+ -Uses a hashmap to store the pool of interned strings. - -Usage example -------------- - -Here's a simple usage example that maps long keys to double values. ------------- -struct hashmap map; - -struct long2double { - struct hashmap_entry ent; /* must be the first member! */ - long key; - double value; -}; - -static int long2double_cmp(const struct long2double *e1, const struct long2double *e2, const void *unused) -{ - return !(e1->key == e2->key); -} - -void long2double_init(void) -{ - hashmap_init(&map, (hashmap_cmp_fn) long2double_cmp, 0); -} - -void long2double_free(void) -{ - hashmap_free(&map, 1); -} - -static struct long2double *find_entry(long key) -{ - struct long2double k; - hashmap_entry_init(&k, memhash(&key, sizeof(long))); - k.key = key; - return hashmap_get(&map, &k, NULL); -} - -double get_value(long key) -{ - struct long2double *e = find_entry(key); - return e ? e->value : 0; -} - -void set_value(long key, double value) -{ - struct long2double *e = find_entry(key); - if (!e) { - e = malloc(sizeof(struct long2double)); - hashmap_entry_init(e, memhash(&key, sizeof(long))); - e->key = key; - hashmap_add(&map, e); - } - e->value = value; -} ------------- - -Using variable-sized keys -------------------------- - -The `hashmap_entry_get` and `hashmap_entry_remove` functions expect an ordinary -`hashmap_entry` structure as key to find the correct entry. If the key data is -variable-sized (e.g. a FLEX_ARRAY string) or quite large, it is undesirable -to create a full-fledged entry structure on the heap and copy all the key data -into the structure. - -In this case, the `keydata` parameter can be used to pass -variable-sized key data directly to the comparison function, and the `key` -parameter can be a stripped-down, fixed size entry structure allocated on the -stack. - -See test-hashmap.c for an example using arbitrary-length strings as keys. diff --git a/Documentation/technical/api-sha1-array.txt b/Documentation/technical/api-oid-array.txt index dcc52943a5..b0c11f868d 100644 --- a/Documentation/technical/api-sha1-array.txt +++ b/Documentation/technical/api-oid-array.txt @@ -1,7 +1,7 @@ -sha1-array API +oid-array API ============== -The sha1-array API provides storage and manipulation of sets of SHA-1 +The oid-array API provides storage and manipulation of sets of object identifiers. The emphasis is on storage and processing efficiency, making them suitable for large lists. Note that the ordering of items is not preserved over some operations. @@ -9,10 +9,10 @@ not preserved over some operations. Data Structures --------------- -`struct sha1_array`:: +`struct oid_array`:: - A single array of SHA-1 hashes. This should be initialized by - assignment from `SHA1_ARRAY_INIT`. The `sha1` member contains + A single array of object IDs. This should be initialized by + assignment from `OID_ARRAY_INIT`. The `oid` member contains the actual data. The `nr` member contains the number of items in the set. The `alloc` and `sorted` members are used internally, and should not be needed by API callers. @@ -20,22 +20,22 @@ Data Structures Functions --------- -`sha1_array_append`:: - Add an item to the set. The sha1 will be placed at the end of +`oid_array_append`:: + Add an item to the set. The object ID will be placed at the end of the array (but note that some operations below may lose this ordering). -`sha1_array_lookup`:: - Perform a binary search of the array for a specific sha1. +`oid_array_lookup`:: + Perform a binary search of the array for a specific object ID. If found, returns the offset (in number of elements) of the - sha1. If not found, returns a negative integer. If the array is - not sorted, this function has the side effect of sorting it. + object ID. If not found, returns a negative integer. If the array + is not sorted, this function has the side effect of sorting it. -`sha1_array_clear`:: +`oid_array_clear`:: Free all memory associated with the array and return it to the initial, empty state. -`sha1_array_for_each_unique`:: +`oid_array_for_each_unique`:: Efficiently iterate over each unique element of the list, executing the callback function for each one. If the array is not sorted, this function has the side effect of sorting it. If @@ -47,25 +47,25 @@ Examples -------- ----------------------------------------- -int print_callback(const unsigned char sha1[20], +int print_callback(const struct object_id *oid, void *data) { - printf("%s\n", sha1_to_hex(sha1)); + printf("%s\n", oid_to_hex(oid)); return 0; /* always continue */ } void some_func(void) { - struct sha1_array hashes = SHA1_ARRAY_INIT; - unsigned char sha1[20]; + struct sha1_array hashes = OID_ARRAY_INIT; + struct object_id oid; /* Read objects into our set */ - while (read_object_from_stdin(sha1)) - sha1_array_append(&hashes, sha1); + while (read_object_from_stdin(oid.hash)) + oid_array_append(&hashes, &oid); /* Check if some objects are in our set */ - while (read_object_from_stdin(sha1)) { - if (sha1_array_lookup(&hashes, sha1) >= 0) + while (read_object_from_stdin(oid.hash)) { + if (oid_array_lookup(&hashes, &oid) >= 0) printf("it's in there!\n"); /* @@ -75,6 +75,6 @@ void some_func(void) * Instead, this will sort once and then skip duplicates * in linear time. */ - sha1_array_for_each_unique(&hashes, print_callback, NULL); + oid_array_for_each_unique(&hashes, print_callback, NULL); } ----------------------------------------- diff --git a/Documentation/technical/api-parse-options.txt b/Documentation/technical/api-parse-options.txt index 27bd701c0d..829b558110 100644 --- a/Documentation/technical/api-parse-options.txt +++ b/Documentation/technical/api-parse-options.txt @@ -168,6 +168,11 @@ There are some macros to easily define options: Introduce an option with string argument. The string argument is put into `str_var`. +`OPT_STRING_LIST(short, long, &struct string_list, arg_str, description)`:: + Introduce an option with string argument. + The string argument is stored as an element in `string_list`. + Use of `--no-option` will clear the list of preceding values. + `OPT_INTEGER(short, long, &int_var, description)`:: Introduce an option with integer argument. The integer is put into `int_var`. @@ -178,13 +183,13 @@ There are some macros to easily define options: scale the provided value by 1024, 1024^2 or 1024^3 respectively. The scaled value is put into `unsigned_long_var`. -`OPT_DATE(short, long, &int_var, description)`:: +`OPT_DATE(short, long, ×tamp_t_var, description)`:: Introduce an option with date argument, see `approxidate()`. - The timestamp is put into `int_var`. + The timestamp is put into `timestamp_t_var`. -`OPT_EXPIRY_DATE(short, long, &int_var, description)`:: +`OPT_EXPIRY_DATE(short, long, ×tamp_t_var, description)`:: Introduce an option with expiry date argument, see `parse_expiry_date()`. - The timestamp is put into `int_var`. + The timestamp is put into `timestamp_t_var`. `OPT_CALLBACK(short, long, &var, arg_str, description, func_ptr)`:: Introduce an option with argument. diff --git a/Documentation/technical/api-ref-iteration.txt b/Documentation/technical/api-ref-iteration.txt index 37379d8337..46c3d5c355 100644 --- a/Documentation/technical/api-ref-iteration.txt +++ b/Documentation/technical/api-ref-iteration.txt @@ -32,11 +32,8 @@ Iteration functions * `for_each_glob_ref_in()` the previous and `for_each_ref_in()` combined. -* `head_ref_submodule()`, `for_each_ref_submodule()`, - `for_each_ref_in_submodule()`, `for_each_tag_ref_submodule()`, - `for_each_branch_ref_submodule()`, `for_each_remote_ref_submodule()` - do the same as the functions described above but for a specified - submodule. +* Use `refs_` API for accessing submodules. The submodule ref store could + be obtained with `get_submodule_ref_store()`. * `for_each_rawref()` can be used to learn about broken ref and symref. diff --git a/Documentation/technical/api-string-list.txt b/Documentation/technical/api-string-list.txt deleted file mode 100644 index c08402b12e..0000000000 --- a/Documentation/technical/api-string-list.txt +++ /dev/null @@ -1,209 +0,0 @@ -string-list API -=============== - -The string_list API offers a data structure and functions to handle -sorted and unsorted string lists. A "sorted" list is one whose -entries are sorted by string value in `strcmp()` order. - -The 'string_list' struct used to be called 'path_list', but was renamed -because it is not specific to paths. - -The caller: - -. Allocates and clears a `struct string_list` variable. - -. Initializes the members. You might want to set the flag `strdup_strings` - if the strings should be strdup()ed. For example, this is necessary - when you add something like git_path("..."), since that function returns - a static buffer that will change with the next call to git_path(). -+ -If you need something advanced, you can manually malloc() the `items` -member (you need this if you add things later) and you should set the -`nr` and `alloc` members in that case, too. - -. Adds new items to the list, using `string_list_append`, - `string_list_append_nodup`, `string_list_insert`, - `string_list_split`, and/or `string_list_split_in_place`. - -. Can check if a string is in the list using `string_list_has_string` or - `unsorted_string_list_has_string` and get it from the list using - `string_list_lookup` for sorted lists. - -. Can sort an unsorted list using `string_list_sort`. - -. Can remove duplicate items from a sorted list using - `string_list_remove_duplicates`. - -. Can remove individual items of an unsorted list using - `unsorted_string_list_delete_item`. - -. Can remove items not matching a criterion from a sorted or unsorted - list using `filter_string_list`, or remove empty strings using - `string_list_remove_empty_items`. - -. Finally it should free the list using `string_list_clear`. - -Example: - ----- -struct string_list list = STRING_LIST_INIT_NODUP; -int i; - -string_list_append(&list, "foo"); -string_list_append(&list, "bar"); -for (i = 0; i < list.nr; i++) - printf("%s\n", list.items[i].string) ----- - -NOTE: It is more efficient to build an unsorted list and sort it -afterwards, instead of building a sorted list (`O(n log n)` instead of -`O(n^2)`). -+ -However, if you use the list to check if a certain string was added -already, you should not do that (using unsorted_string_list_has_string()), -because the complexity would be quadratic again (but with a worse factor). - -Functions ---------- - -* General ones (works with sorted and unsorted lists as well) - -`string_list_init`:: - - Initialize the members of the string_list, set `strdup_strings` - member according to the value of the second parameter. - -`filter_string_list`:: - - Apply a function to each item in a list, retaining only the - items for which the function returns true. If free_util is - true, call free() on the util members of any items that have - to be deleted. Preserve the order of the items that are - retained. - -`string_list_remove_empty_items`:: - - Remove any empty strings from the list. If free_util is true, - call free() on the util members of any items that have to be - deleted. Preserve the order of the items that are retained. - -`print_string_list`:: - - Dump a string_list to stdout, useful mainly for debugging purposes. It - can take an optional header argument and it writes out the - string-pointer pairs of the string_list, each one in its own line. - -`string_list_clear`:: - - Free a string_list. The `string` pointer of the items will be freed in - case the `strdup_strings` member of the string_list is set. The second - parameter controls if the `util` pointer of the items should be freed - or not. - -* Functions for sorted lists only - -`string_list_has_string`:: - - Determine if the string_list has a given string or not. - -`string_list_insert`:: - - Insert a new element to the string_list. The returned pointer can be - handy if you want to write something to the `util` pointer of the - string_list_item containing the just added string. If the given - string already exists the insertion will be skipped and the - pointer to the existing item returned. -+ -Since this function uses xrealloc() (which die()s if it fails) if the -list needs to grow, it is safe not to check the pointer. I.e. you may -write `string_list_insert(...)->util = ...;`. - -`string_list_lookup`:: - - Look up a given string in the string_list, returning the containing - string_list_item. If the string is not found, NULL is returned. - -`string_list_remove_duplicates`:: - - Remove all but the first of consecutive entries that have the - same string value. If free_util is true, call free() on the - util members of any items that have to be deleted. - -* Functions for unsorted lists only - -`string_list_append`:: - - Append a new string to the end of the string_list. If - `strdup_string` is set, then the string argument is copied; - otherwise the new `string_list_entry` refers to the input - string. - -`string_list_append_nodup`:: - - Append a new string to the end of the string_list. The new - `string_list_entry` always refers to the input string, even if - `strdup_string` is set. This function can be used to hand - ownership of a malloc()ed string to a `string_list` that has - `strdup_string` set. - -`string_list_sort`:: - - Sort the list's entries by string value in `strcmp()` order. - -`unsorted_string_list_has_string`:: - - It's like `string_list_has_string()` but for unsorted lists. - -`unsorted_string_list_lookup`:: - - It's like `string_list_lookup()` but for unsorted lists. -+ -The above two functions need to look through all items, as opposed to their -counterpart for sorted lists, which performs a binary search. - -`unsorted_string_list_delete_item`:: - - Remove an item from a string_list. The `string` pointer of the items - will be freed in case the `strdup_strings` member of the string_list - is set. The third parameter controls if the `util` pointer of the - items should be freed or not. - -`string_list_split`:: -`string_list_split_in_place`:: - - Split a string into substrings on a delimiter character and - append the substrings to a `string_list`. If `maxsplit` is - non-negative, then split at most `maxsplit` times. Return the - number of substrings appended to the list. -+ -`string_list_split` requires a `string_list` that has `strdup_strings` -set to true; it leaves the input string untouched and makes copies of -the substrings in newly-allocated memory. -`string_list_split_in_place` requires a `string_list` that has -`strdup_strings` set to false; it splits the input string in place, -overwriting the delimiter characters with NULs and creating new -string_list_items that point into the original string (the original -string must therefore not be modified or freed while the `string_list` -is in use). - - -Data structures ---------------- - -* `struct string_list_item` - -Represents an item of the list. The `string` member is a pointer to the -string, and you may use the `util` member for any purpose, if you want. - -* `struct string_list` - -Represents the list itself. - -. The array of items are available via the `items` member. -. The `nr` member contains the number of items stored in the list. -. The `alloc` member is used to avoid reallocating at every insertion. - You should not tamper with it. -. Setting the `strdup_strings` member to 1 will strdup() the strings - before adding them, see above. -. The `compare_strings_fn` member is used to specify a custom compare - function, otherwise `strcmp()` is used as the default function. diff --git a/Documentation/technical/api-tree-walking.txt b/Documentation/technical/api-tree-walking.txt index 14af37c3f1..bde18622a8 100644 --- a/Documentation/technical/api-tree-walking.txt +++ b/Documentation/technical/api-tree-walking.txt @@ -55,9 +55,9 @@ Initializing `fill_tree_descriptor`:: - Initialize a `tree_desc` and decode its first entry given the sha1 of - a tree. Returns the `buffer` member if the sha1 is a valid tree - identifier and NULL otherwise. + Initialize a `tree_desc` and decode its first entry given the + object ID of a tree. Returns the `buffer` member if the latter + is a valid tree identifier and NULL otherwise. `setup_traverse_info`:: diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt new file mode 100644 index 0000000000..417ba491d0 --- /dev/null +++ b/Documentation/technical/hash-function-transition.txt @@ -0,0 +1,797 @@ +Git hash function transition +============================ + +Objective +--------- +Migrate Git from SHA-1 to a stronger hash function. + +Background +---------- +At its core, the Git version control system is a content addressable +filesystem. It uses the SHA-1 hash function to name content. For +example, files, directories, and revisions are referred to by hash +values unlike in other traditional version control systems where files +or versions are referred to via sequential numbers. The use of a hash +function to address its content delivers a few advantages: + +* Integrity checking is easy. Bit flips, for example, are easily + detected, as the hash of corrupted content does not match its name. +* Lookup of objects is fast. + +Using a cryptographically secure hash function brings additional +advantages: + +* Object names can be signed and third parties can trust the hash to + address the signed object and all objects it references. +* Communication using Git protocol and out of band communication + methods have a short reliable string that can be used to reliably + address stored content. + +Over time some flaws in SHA-1 have been discovered by security +researchers. https://shattered.io demonstrated a practical SHA-1 hash +collision. As a result, SHA-1 cannot be considered cryptographically +secure any more. This impacts the communication of hash values because +we cannot trust that a given hash value represents the known good +version of content that the speaker intended. + +SHA-1 still possesses the other properties such as fast object lookup +and safe error checking, but other hash functions are equally suitable +that are believed to be cryptographically secure. + +Goals +----- +Where NewHash is a strong 256-bit hash function to replace SHA-1 (see +"Selection of a New Hash", below): + +1. The transition to NewHash can be done one local repository at a time. + a. Requiring no action by any other party. + b. A NewHash repository can communicate with SHA-1 Git servers + (push/fetch). + c. Users can use SHA-1 and NewHash identifiers for objects + interchangeably (see "Object names on the command line", below). + d. New signed objects make use of a stronger hash function than + SHA-1 for their security guarantees. +2. Allow a complete transition away from SHA-1. + a. Local metadata for SHA-1 compatibility can be removed from a + repository if compatibility with SHA-1 is no longer needed. +3. Maintainability throughout the process. + a. The object format is kept simple and consistent. + b. Creation of a generalized repository conversion tool. + +Non-Goals +--------- +1. Add NewHash support to Git protocol. This is valuable and the + logical next step but it is out of scope for this initial design. +2. Transparently improving the security of existing SHA-1 signed + objects. +3. Intermixing objects using multiple hash functions in a single + repository. +4. Taking the opportunity to fix other bugs in Git's formats and + protocols. +5. Shallow clones and fetches into a NewHash repository. (This will + change when we add NewHash support to Git protocol.) +6. Skip fetching some submodules of a project into a NewHash + repository. (This also depends on NewHash support in Git + protocol.) + +Overview +-------- +We introduce a new repository format extension. Repositories with this +extension enabled use NewHash instead of SHA-1 to name their objects. +This affects both object names and object content --- both the names +of objects and all references to other objects within an object are +switched to the new hash function. + +NewHash repositories cannot be read by older versions of Git. + +Alongside the packfile, a NewHash repository stores a bidirectional +mapping between NewHash and SHA-1 object names. The mapping is generated +locally and can be verified using "git fsck". Object lookups use this +mapping to allow naming objects using either their SHA-1 and NewHash names +interchangeably. + +"git cat-file" and "git hash-object" gain options to display an object +in its sha1 form and write an object given its sha1 form. This +requires all objects referenced by that object to be present in the +object database so that they can be named using the appropriate name +(using the bidirectional hash mapping). + +Fetches from a SHA-1 based server convert the fetched objects into +NewHash form and record the mapping in the bidirectional mapping table +(see below for details). Pushes to a SHA-1 based server convert the +objects being pushed into sha1 form so the server does not have to be +aware of the hash function the client is using. + +Detailed Design +--------------- +Repository format extension +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +A NewHash repository uses repository format version `1` (see +Documentation/technical/repository-version.txt) with extensions +`objectFormat` and `compatObjectFormat`: + + [core] + repositoryFormatVersion = 1 + [extensions] + objectFormat = newhash + compatObjectFormat = sha1 + +Specifying a repository format extension ensures that versions of Git +not aware of NewHash do not try to operate on these repositories, +instead producing an error message: + + $ git status + fatal: unknown repository extensions found: + objectformat + compatobjectformat + +See the "Transition plan" section below for more details on these +repository extensions. + +Object names +~~~~~~~~~~~~ +Objects can be named by their 40 hexadecimal digit sha1-name or 64 +hexadecimal digit newhash-name, plus names derived from those (see +gitrevisions(7)). + +The sha1-name of an object is the SHA-1 of the concatenation of its +type, length, a nul byte, and the object's sha1-content. This is the +traditional <sha1> used in Git to name objects. + +The newhash-name of an object is the NewHash of the concatenation of its +type, length, a nul byte, and the object's newhash-content. + +Object format +~~~~~~~~~~~~~ +The content as a byte sequence of a tag, commit, or tree object named +by sha1 and newhash differ because an object named by newhash-name refers to +other objects by their newhash-names and an object named by sha1-name +refers to other objects by their sha1-names. + +The newhash-content of an object is the same as its sha1-content, except +that objects referenced by the object are named using their newhash-names +instead of sha1-names. Because a blob object does not refer to any +other object, its sha1-content and newhash-content are the same. + +The format allows round-trip conversion between newhash-content and +sha1-content. + +Object storage +~~~~~~~~~~~~~~ +Loose objects use zlib compression and packed objects use the packed +format described in Documentation/technical/pack-format.txt, just like +today. The content that is compressed and stored uses newhash-content +instead of sha1-content. + +Pack index +~~~~~~~~~~ +Pack index (.idx) files use a new v3 format that supports multiple +hash functions. They have the following format (all integers are in +network byte order): + +- A header appears at the beginning and consists of the following: + - The 4-byte pack index signature: '\377t0c' + - 4-byte version number: 3 + - 4-byte length of the header section, including the signature and + version number + - 4-byte number of objects contained in the pack + - 4-byte number of object formats in this pack index: 2 + - For each object format: + - 4-byte format identifier (e.g., 'sha1' for SHA-1) + - 4-byte length in bytes of shortened object names. This is the + shortest possible length needed to make names in the shortened + object name table unambiguous. + - 4-byte integer, recording where tables relating to this format + are stored in this index file, as an offset from the beginning. + - 4-byte offset to the trailer from the beginning of this file. + - Zero or more additional key/value pairs (4-byte key, 4-byte + value). Only one key is supported: 'PSRC'. See the "Loose objects + and unreachable objects" section for supported values and how this + is used. All other keys are reserved. Readers must ignore + unrecognized keys. +- Zero or more NUL bytes. This can optionally be used to improve the + alignment of the full object name table below. +- Tables for the first object format: + - A sorted table of shortened object names. These are prefixes of + the names of all objects in this pack file, packed together + without offset values to reduce the cache footprint of the binary + search for a specific object name. + + - A table of full object names in pack order. This allows resolving + a reference to "the nth object in the pack file" (from a + reachability bitmap or from the next table of another object + format) to its object name. + + - A table of 4-byte values mapping object name order to pack order. + For an object in the table of sorted shortened object names, the + value at the corresponding index in this table is the index in the + previous table for that same object. + + This can be used to look up the object in reachability bitmaps or + to look up its name in another object format. + + - A table of 4-byte CRC32 values of the packed object data, in the + order that the objects appear in the pack file. This is to allow + compressed data to be copied directly from pack to pack during + repacking without undetected data corruption. + + - A table of 4-byte offset values. For an object in the table of + sorted shortened object names, the value at the corresponding + index in this table indicates where that object can be found in + the pack file. These are usually 31-bit pack file offsets, but + large offsets are encoded as an index into the next table with the + most significant bit set. + + - A table of 8-byte offset entries (empty for pack files less than + 2 GiB). Pack files are organized with heavily used objects toward + the front, so most object references should not need to refer to + this table. +- Zero or more NUL bytes. +- Tables for the second object format, with the same layout as above, + up to and not including the table of CRC32 values. +- Zero or more NUL bytes. +- The trailer consists of the following: + - A copy of the 20-byte NewHash checksum at the end of the + corresponding packfile. + + - 20-byte NewHash checksum of all of the above. + +Loose object index +~~~~~~~~~~~~~~~~~~ +A new file $GIT_OBJECT_DIR/loose-object-idx contains information about +all loose objects. Its format is + + # loose-object-idx + (newhash-name SP sha1-name LF)* + +where the object names are in hexadecimal format. The file is not +sorted. + +The loose object index is protected against concurrent writes by a +lock file $GIT_OBJECT_DIR/loose-object-idx.lock. To add a new loose +object: + +1. Write the loose object to a temporary file, like today. +2. Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the lock. +3. Rename the loose object into place. +4. Open loose-object-idx with O_APPEND and write the new object +5. Unlink loose-object-idx.lock to release the lock. + +To remove entries (e.g. in "git pack-refs" or "git-prune"): + +1. Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the + lock. +2. Write the new content to loose-object-idx.lock. +3. Unlink any loose objects being removed. +4. Rename to replace loose-object-idx, releasing the lock. + +Translation table +~~~~~~~~~~~~~~~~~ +The index files support a bidirectional mapping between sha1-names +and newhash-names. The lookup proceeds similarly to ordinary object +lookups. For example, to convert a sha1-name to a newhash-name: + + 1. Look for the object in idx files. If a match is present in the + idx's sorted list of truncated sha1-names, then: + a. Read the corresponding entry in the sha1-name order to pack + name order mapping. + b. Read the corresponding entry in the full sha1-name table to + verify we found the right object. If it is, then + c. Read the corresponding entry in the full newhash-name table. + That is the object's newhash-name. + 2. Check for a loose object. Read lines from loose-object-idx until + we find a match. + +Step (1) takes the same amount of time as an ordinary object lookup: +O(number of packs * log(objects per pack)). Step (2) takes O(number of +loose objects) time. To maintain good performance it will be necessary +to keep the number of loose objects low. See the "Loose objects and +unreachable objects" section below for more details. + +Since all operations that make new objects (e.g., "git commit") add +the new objects to the corresponding index, this mapping is possible +for all objects in the object store. + +Reading an object's sha1-content +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The sha1-content of an object can be read by converting all newhash-names +its newhash-content references to sha1-names using the translation table. + +Fetch +~~~~~ +Fetching from a SHA-1 based server requires translating between SHA-1 +and NewHash based representations on the fly. + +SHA-1s named in the ref advertisement that are present on the client +can be translated to NewHash and looked up as local objects using the +translation table. + +Negotiation proceeds as today. Any "have"s generated locally are +converted to SHA-1 before being sent to the server, and SHA-1s +mentioned by the server are converted to NewHash when looking them up +locally. + +After negotiation, the server sends a packfile containing the +requested objects. We convert the packfile to NewHash format using +the following steps: + +1. index-pack: inflate each object in the packfile and compute its + SHA-1. Objects can contain deltas in OBJ_REF_DELTA format against + objects the client has locally. These objects can be looked up + using the translation table and their sha1-content read as + described above to resolve the deltas. +2. topological sort: starting at the "want"s from the negotiation + phase, walk through objects in the pack and emit a list of them, + excluding blobs, in reverse topologically sorted order, with each + object coming later in the list than all objects it references. + (This list only contains objects reachable from the "wants". If the + pack from the server contained additional extraneous objects, then + they will be discarded.) +3. convert to newhash: open a new (newhash) packfile. Read the topologically + sorted list just generated. For each object, inflate its + sha1-content, convert to newhash-content, and write it to the newhash + pack. Record the new sha1<->newhash mapping entry for use in the idx. +4. sort: reorder entries in the new pack to match the order of objects + in the pack the server generated and include blobs. Write a newhash idx + file +5. clean up: remove the SHA-1 based pack file, index, and + topologically sorted list obtained from the server in steps 1 + and 2. + +Step 3 requires every object referenced by the new object to be in the +translation table. This is why the topological sort step is necessary. + +As an optimization, step 1 could write a file describing what non-blob +objects each object it has inflated from the packfile references. This +makes the topological sort in step 2 possible without inflating the +objects in the packfile for a second time. The objects need to be +inflated again in step 3, for a total of two inflations. + +Step 4 is probably necessary for good read-time performance. "git +pack-objects" on the server optimizes the pack file for good data +locality (see Documentation/technical/pack-heuristics.txt). + +Details of this process are likely to change. It will take some +experimenting to get this to perform well. + +Push +~~~~ +Push is simpler than fetch because the objects referenced by the +pushed objects are already in the translation table. The sha1-content +of each object being pushed can be read as described in the "Reading +an object's sha1-content" section to generate the pack written by git +send-pack. + +Signed Commits +~~~~~~~~~~~~~~ +We add a new field "gpgsig-newhash" to the commit object format to allow +signing commits without relying on SHA-1. It is similar to the +existing "gpgsig" field. Its signed payload is the newhash-content of the +commit object with any "gpgsig" and "gpgsig-newhash" fields removed. + +This means commits can be signed +1. using SHA-1 only, as in existing signed commit objects +2. using both SHA-1 and NewHash, by using both gpgsig-newhash and gpgsig + fields. +3. using only NewHash, by only using the gpgsig-newhash field. + +Old versions of "git verify-commit" can verify the gpgsig signature in +cases (1) and (2) without modifications and view case (3) as an +ordinary unsigned commit. + +Signed Tags +~~~~~~~~~~~ +We add a new field "gpgsig-newhash" to the tag object format to allow +signing tags without relying on SHA-1. Its signed payload is the +newhash-content of the tag with its gpgsig-newhash field and "-----BEGIN PGP +SIGNATURE-----" delimited in-body signature removed. + +This means tags can be signed +1. using SHA-1 only, as in existing signed tag objects +2. using both SHA-1 and NewHash, by using gpgsig-newhash and an in-body + signature. +3. using only NewHash, by only using the gpgsig-newhash field. + +Mergetag embedding +~~~~~~~~~~~~~~~~~~ +The mergetag field in the sha1-content of a commit contains the +sha1-content of a tag that was merged by that commit. + +The mergetag field in the newhash-content of the same commit contains the +newhash-content of the same tag. + +Submodules +~~~~~~~~~~ +To convert recorded submodule pointers, you need to have the converted +submodule repository in place. The translation table of the submodule +can be used to look up the new hash. + +Loose objects and unreachable objects +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Fast lookups in the loose-object-idx require that the number of loose +objects not grow too high. + +"git gc --auto" currently waits for there to be 6700 loose objects +present before consolidating them into a packfile. We will need to +measure to find a more appropriate threshold for it to use. + +"git gc --auto" currently waits for there to be 50 packs present +before combining packfiles. Packing loose objects more aggressively +may cause the number of pack files to grow too quickly. This can be +mitigated by using a strategy similar to Martin Fick's exponential +rolling garbage collection script: +https://gerrit-review.googlesource.com/c/gerrit/+/35215 + +"git gc" currently expels any unreachable objects it encounters in +pack files to loose objects in an attempt to prevent a race when +pruning them (in case another process is simultaneously writing a new +object that refers to the about-to-be-deleted object). This leads to +an explosion in the number of loose objects present and disk space +usage due to the objects in delta form being replaced with independent +loose objects. Worse, the race is still present for loose objects. + +Instead, "git gc" will need to move unreachable objects to a new +packfile marked as UNREACHABLE_GARBAGE (using the PSRC field; see +below). To avoid the race when writing new objects referring to an +about-to-be-deleted object, code paths that write new objects will +need to copy any objects from UNREACHABLE_GARBAGE packs that they +refer to to new, non-UNREACHABLE_GARBAGE packs (or loose objects). +UNREACHABLE_GARBAGE are then safe to delete if their creation time (as +indicated by the file's mtime) is long enough ago. + +To avoid a proliferation of UNREACHABLE_GARBAGE packs, they can be +combined under certain circumstances. If "gc.garbageTtl" is set to +greater than one day, then packs created within a single calendar day, +UTC, can be coalesced together. The resulting packfile would have an +mtime before midnight on that day, so this makes the effective maximum +ttl the garbageTtl + 1 day. If "gc.garbageTtl" is less than one day, +then we divide the calendar day into intervals one-third of that ttl +in duration. Packs created within the same interval can be coalesced +together. The resulting packfile would have an mtime before the end of +the interval, so this makes the effective maximum ttl equal to the +garbageTtl * 4/3. + +This rule comes from Thirumala Reddy Mutchukota's JGit change +https://git.eclipse.org/r/90465. + +The UNREACHABLE_GARBAGE setting goes in the PSRC field of the pack +index. More generally, that field indicates where a pack came from: + + - 1 (PACK_SOURCE_RECEIVE) for a pack received over the network + - 2 (PACK_SOURCE_AUTO) for a pack created by a lightweight + "gc --auto" operation + - 3 (PACK_SOURCE_GC) for a pack created by a full gc + - 4 (PACK_SOURCE_UNREACHABLE_GARBAGE) for potential garbage + discovered by gc + - 5 (PACK_SOURCE_INSERT) for locally created objects that were + written directly to a pack file, e.g. from "git add ." + +This information can be useful for debugging and for "gc --auto" to +make appropriate choices about which packs to coalesce. + +Caveats +------- +Invalid objects +~~~~~~~~~~~~~~~ +The conversion from sha1-content to newhash-content retains any +brokenness in the original object (e.g., tree entry modes encoded with +leading 0, tree objects whose paths are not sorted correctly, and +commit objects without an author or committer). This is a deliberate +feature of the design to allow the conversion to round-trip. + +More profoundly broken objects (e.g., a commit with a truncated "tree" +header line) cannot be converted but were not usable by current Git +anyway. + +Shallow clone and submodules +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Because it requires all referenced objects to be available in the +locally generated translation table, this design does not support +shallow clone or unfetched submodules. Protocol improvements might +allow lifting this restriction. + +Alternates +~~~~~~~~~~ +For the same reason, a newhash repository cannot borrow objects from a +sha1 repository using objects/info/alternates or +$GIT_ALTERNATE_OBJECT_REPOSITORIES. + +git notes +~~~~~~~~~ +The "git notes" tool annotates objects using their sha1-name as key. +This design does not describe a way to migrate notes trees to use +newhash-names. That migration is expected to happen separately (for +example using a file at the root of the notes tree to describe which +hash it uses). + +Server-side cost +~~~~~~~~~~~~~~~~ +Until Git protocol gains NewHash support, using NewHash based storage +on public-facing Git servers is strongly discouraged. Once Git +protocol gains NewHash support, NewHash based servers are likely not +to support SHA-1 compatibility, to avoid what may be a very expensive +hash reencode during clone and to encourage peers to modernize. + +The design described here allows fetches by SHA-1 clients of a +personal NewHash repository because it's not much more difficult than +allowing pushes from that repository. This support needs to be guarded +by a configuration option --- servers like git.kernel.org that serve a +large number of clients would not be expected to bear that cost. + +Meaning of signatures +~~~~~~~~~~~~~~~~~~~~~ +The signed payload for signed commits and tags does not explicitly +name the hash used to identify objects. If some day Git adopts a new +hash function with the same length as the current SHA-1 (40 +hexadecimal digit) or NewHash (64 hexadecimal digit) objects then the +intent behind the PGP signed payload in an object signature is +unclear: + + object e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 + type commit + tag v2.12.0 + tagger Junio C Hamano <gitster@pobox.com> 1487962205 -0800 + + Git 2.12 + +Does this mean Git v2.12.0 is the commit with sha1-name +e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with +new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7? + +Fortunately NewHash and SHA-1 have different lengths. If Git starts +using another hash with the same length to name objects, then it will +need to change the format of signed payloads using that hash to +address this issue. + +Object names on the command line +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +To support the transition (see Transition plan below), this design +supports four different modes of operation: + + 1. ("dark launch") Treat object names input by the user as SHA-1 and + convert any object names written to output to SHA-1, but store + objects using NewHash. This allows users to test the code with no + visible behavior change except for performance. This allows + allows running even tests that assume the SHA-1 hash function, to + sanity-check the behavior of the new mode. + + 2. ("early transition") Allow both SHA-1 and NewHash object names in + input. Any object names written to output use SHA-1. This allows + users to continue to make use of SHA-1 to communicate with peers + (e.g. by email) that have not migrated yet and prepares for mode 3. + + 3. ("late transition") Allow both SHA-1 and NewHash object names in + input. Any object names written to output use NewHash. In this + mode, users are using a more secure object naming method by + default. The disruption is minimal as long as most of their peers + are in mode 2 or mode 3. + + 4. ("post-transition") Treat object names input by the user as + NewHash and write output using NewHash. This is safer than mode 3 + because there is less risk that input is incorrectly interpreted + using the wrong hash function. + +The mode is specified in configuration. + +The user can also explicitly specify which format to use for a +particular revision specifier and for output, overriding the mode. For +example: + +git --output-format=sha1 log abac87a^{sha1}..f787cac^{newhash} + +Selection of a New Hash +----------------------- +In early 2005, around the time that Git was written, Xiaoyun Wang, +Yiqun Lisa Yin, and Hongbo Yu announced an attack finding SHA-1 +collisions in 2^69 operations. In August they published details. +Luckily, no practical demonstrations of a collision in full SHA-1 were +published until 10 years later, in 2017. + +The hash function NewHash to replace SHA-1 should be stronger than +SHA-1 was: we would like it to be trustworthy and useful in practice +for at least 10 years. + +Some other relevant properties: + +1. A 256-bit hash (long enough to match common security practice; not + excessively long to hurt performance and disk usage). + +2. High quality implementations should be widely available (e.g. in + OpenSSL). + +3. The hash function's properties should match Git's needs (e.g. Git + requires collision and 2nd preimage resistance and does not require + length extension resistance). + +4. As a tiebreaker, the hash should be fast to compute (fortunately + many contenders are faster than SHA-1). + +Some hashes under consideration are SHA-256, SHA-512/256, SHA-256x16, +K12, and BLAKE2bp-256. + +Transition plan +--------------- +Some initial steps can be implemented independently of one another: +- adding a hash function API (vtable) +- teaching fsck to tolerate the gpgsig-newhash field +- excluding gpgsig-* from the fields copied by "git commit --amend" +- annotating tests that depend on SHA-1 values with a SHA1 test + prerequisite +- using "struct object_id", GIT_MAX_RAWSZ, and GIT_MAX_HEXSZ + consistently instead of "unsigned char *" and the hardcoded + constants 20 and 40. +- introducing index v3 +- adding support for the PSRC field and safer object pruning + + +The first user-visible change is the introduction of the objectFormat +extension (without compatObjectFormat). This requires: +- implementing the loose-object-idx +- teaching fsck about this mode of operation +- using the hash function API (vtable) when computing object names +- signing objects and verifying signatures +- rejecting attempts to fetch from or push to an incompatible + repository + +Next comes introduction of compatObjectFormat: +- translating object names between object formats +- translating object content between object formats +- generating and verifying signatures in the compat format +- adding appropriate index entries when adding a new object to the + object store +- --output-format option +- ^{sha1} and ^{newhash} revision notation +- configuration to specify default input and output format (see + "Object names on the command line" above) + +The next step is supporting fetches and pushes to SHA-1 repositories: +- allow pushes to a repository using the compat format +- generate a topologically sorted list of the SHA-1 names of fetched + objects +- convert the fetched packfile to newhash format and generate an idx + file +- re-sort to match the order of objects in the fetched packfile + +The infrastructure supporting fetch also allows converting an existing +repository. In converted repositories and new clones, end users can +gain support for the new hash function without any visible change in +behavior (see "dark launch" in the "Object names on the command line" +section). In particular this allows users to verify NewHash signatures +on objects in the repository, and it should ensure the transition code +is stable in production in preparation for using it more widely. + +Over time projects would encourage their users to adopt the "early +transition" and then "late transition" modes to take advantage of the +new, more futureproof NewHash object names. + +When objectFormat and compatObjectFormat are both set, commands +generating signatures would generate both SHA-1 and NewHash signatures +by default to support both new and old users. + +In projects using NewHash heavily, users could be encouraged to adopt +the "post-transition" mode to avoid accidentally making implicit use +of SHA-1 object names. + +Once a critical mass of users have upgraded to a version of Git that +can verify NewHash signatures and have converted their existing +repositories to support verifying them, we can add support for a +setting to generate only NewHash signatures. This is expected to be at +least a year later. + +That is also a good moment to advertise the ability to convert +repositories to use NewHash only, stripping out all SHA-1 related +metadata. This improves performance by eliminating translation +overhead and security by avoiding the possibility of accidentally +relying on the safety of SHA-1. + +Updating Git's protocols to allow a server to specify which hash +functions it supports is also an important part of this transition. It +is not discussed in detail in this document but this transition plan +assumes it happens. :) + +Alternatives considered +----------------------- +Upgrading everyone working on a particular project on a flag day +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Projects like the Linux kernel are large and complex enough that +flipping the switch for all projects based on the repository at once +is infeasible. + +Not only would all developers and server operators supporting +developers have to switch on the same flag day, but supporting tooling +(continuous integration, code review, bug trackers, etc) would have to +be adapted as well. This also makes it difficult to get early feedback +from some project participants testing before it is time for mass +adoption. + +Using hash functions in parallel +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +(e.g. https://public-inbox.org/git/22708.8913.864049.452252@chiark.greenend.org.uk/ ) +Objects newly created would be addressed by the new hash, but inside +such an object (e.g. commit) it is still possible to address objects +using the old hash function. +* You cannot trust its history (needed for bisectability) in the + future without further work +* Maintenance burden as the number of supported hash functions grows + (they will never go away, so they accumulate). In this proposal, by + comparison, converted objects lose all references to SHA-1. + +Signed objects with multiple hashes +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Instead of introducing the gpgsig-newhash field in commit and tag objects +for newhash-content based signatures, an earlier version of this design +added "hash newhash <newhash-name>" fields to strengthen the existing +sha1-content based signatures. + +In other words, a single signature was used to attest to the object +content using both hash functions. This had some advantages: +* Using one signature instead of two speeds up the signing process. +* Having one signed payload with both hashes allows the signer to + attest to the sha1-name and newhash-name referring to the same object. +* All users consume the same signature. Broken signatures are likely + to be detected quickly using current versions of git. + +However, it also came with disadvantages: +* Verifying a signed object requires access to the sha1-names of all + objects it references, even after the transition is complete and + translation table is no longer needed for anything else. To support + this, the design added fields such as "hash sha1 tree <sha1-name>" + and "hash sha1 parent <sha1-name>" to the newhash-content of a signed + commit, complicating the conversion process. +* Allowing signed objects without a sha1 (for after the transition is + complete) complicated the design further, requiring a "nohash sha1" + field to suppress including "hash sha1" fields in the newhash-content + and signed payload. + +Lazily populated translation table +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Some of the work of building the translation table could be deferred to +push time, but that would significantly complicate and slow down pushes. +Calculating the sha1-name at object creation time at the same time it is +being streamed to disk and having its newhash-name calculated should be +an acceptable cost. + +Document History +---------------- + +2017-03-03 +bmwill@google.com, jonathantanmy@google.com, jrnieder@gmail.com, +sbeller@google.com + +Initial version sent to +http://public-inbox.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com + +2017-03-03 jrnieder@gmail.com +Incorporated suggestions from jonathantanmy and sbeller: +* describe purpose of signed objects with each hash type +* redefine signed object verification using object content under the + first hash function + +2017-03-06 jrnieder@gmail.com +* Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2] +* Make sha3-based signatures a separate field, avoiding the need for + "hash" and "nohash" fields (thanks to peff[3]). +* Add a sorting phase to fetch (thanks to Junio for noticing the need + for this). +* Omit blobs from the topological sort during fetch (thanks to peff). +* Discuss alternates, git notes, and git servers in the caveats + section (thanks to Junio Hamano, brian m. carlson[4], and Shawn + Pearce). +* Clarify language throughout (thanks to various commenters, + especially Junio). + +2017-09-27 jrnieder@gmail.com, sbeller@google.com +* use placeholder NewHash instead of SHA3-256 +* describe criteria for picking a hash function. +* include a transition plan (thanks especially to Brandon Williams + for fleshing these ideas out) +* define the translation table (thanks, Shawn Pearce[5], Jonathan + Tan, and Masaya Suzuki) +* avoid loose object overhead by packing more aggressively in + "git gc --auto" + +[1] http://public-inbox.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/ +[2] http://public-inbox.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/ +[3] http://public-inbox.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/ +[4] http://public-inbox.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net +[5] https://public-inbox.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/ diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt index ade0b0c445..db3572626b 100644 --- a/Documentation/technical/index-format.txt +++ b/Documentation/technical/index-format.txt @@ -295,3 +295,22 @@ The remaining data of each directory block is grouped by type: in the previous ewah bitmap. - One NUL. + +== File System Monitor cache + + The file system monitor cache tracks files for which the core.fsmonitor + hook has told us about changes. The signature for this extension is + { 'F', 'S', 'M', 'N' }. + + The extension starts with + + - 32-bit version number: the current supported version is 1. + + - 64-bit time: the extension data reflects all changes through the given + time which is stored as the nanoseconds elapsed since midnight, + January 1, 1970. + + - 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap. + + - An ewah bitmap, the n-th bit indicates whether the n-th index entry + is not CE_FSMONITOR_VALID. diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt index c59ac9936a..ed1eae8b83 100644 --- a/Documentation/technical/pack-protocol.txt +++ b/Documentation/technical/pack-protocol.txt @@ -199,7 +199,7 @@ After reference and capabilities discovery, the client can decide to terminate the connection by sending a flush-pkt, telling the server it can now gracefully terminate, and disconnect, when it does not need any pack data. This can happen with the ls-remote command, and also can happen when -the client already is up-to-date. +the client already is up to date. Otherwise, it enters the negotiation phase, where the client and server determine what the minimal packfile necessary for transport is, @@ -351,14 +351,19 @@ ACK after 'done' if there is at least one common base and multi_ack or multi_ack_detailed is enabled. The server always sends NAK after 'done' if there is no common base found. +Instead of 'ACK' or 'NAK', the server may send an error message (for +example, if it does not recognize an object in a 'want' line received +from the client). + Then the server will start sending its packfile data. ---- - server-response = *ack_multi ack / nak + server-response = *ack_multi ack / nak / error-line ack_multi = PKT-LINE("ACK" SP obj-id ack_status) ack_status = "continue" / "common" / "ready" ack = PKT-LINE("ACK" SP obj-id) nak = PKT-LINE("NAK") + error-line = PKT-LINE("ERR" SP explanation-text) ---- A simple clone may look like this (with no 'have' lines): @@ -468,13 +473,10 @@ that it wants to update, it sends a line listing the obj-id currently on the server, the obj-id the client would like to update it to and the name of the reference. -This list is followed by a flush-pkt. Then the push options are transmitted -one per packet followed by another flush-pkt. After that the packfile that -should contain all the objects that the server will need to complete the new -references will be sent. +This list is followed by a flush-pkt. ---- - update-request = *shallow ( command-list | push-cert ) [packfile] + update-requests = *shallow ( command-list | push-cert ) shallow = PKT-LINE("shallow" SP obj-id) @@ -495,12 +497,35 @@ references will be sent. PKT-LINE("pusher" SP ident LF) PKT-LINE("pushee" SP url LF) PKT-LINE("nonce" SP nonce LF) + *PKT-LINE("push-option" SP push-option LF) PKT-LINE(LF) *PKT-LINE(command LF) *PKT-LINE(gpg-signature-lines LF) PKT-LINE("push-cert-end" LF) - packfile = "PACK" 28*(OCTET) + push-option = 1*( VCHAR | SP ) +---- + +If the server has advertised the 'push-options' capability and the client has +specified 'push-options' as part of the capability list above, the client then +sends its push options followed by a flush-pkt. + +---- + push-options = *PKT-LINE(push-option) flush-pkt +---- + +For backwards compatibility with older Git servers, if the client sends a push +cert and push options, it MUST send its push options both embedded within the +push cert and after the push cert. (Note that the push options within the cert +are prefixed, but the push options after the cert are not.) Both these lists +MUST be the same, modulo the prefix. + +After that the packfile that +should contain all the objects that the server will need to complete the new +references will be sent. + +---- + packfile = "PACK" 28*(OCTET) ---- If the receiving end does not support delete-refs, the sending end MUST diff --git a/Documentation/technical/trivial-merge.txt b/Documentation/technical/trivial-merge.txt index c79d4a7c47..1f1c33d0da 100644 --- a/Documentation/technical/trivial-merge.txt +++ b/Documentation/technical/trivial-merge.txt @@ -32,7 +32,7 @@ or the result. If multiple cases apply, the one used is listed first. A result which changes the index is an error if the index is not empty -and not up-to-date. +and not up to date. Entries marked '+' have stat information. Spaces marked '*' don't affect the result. @@ -65,7 +65,7 @@ empty, no entry is left for that stage). Otherwise, the given entry is left in stage 0, and there are no other entries. A result of "no merge" is an error if the index is not empty and not -up-to-date. +up to date. *empty* means that the tree must not have a directory-file conflict with the entry. |